Because the crystal growth of nitride semiconductors in a bulk form from its melt is generally difficult, a self-supported nitride semiconductor substrate is obtained by growing nitride semiconductor layer on a different base substrate such as sapphire and gallium arsenide, and removing layers ranging from the base substrate to leave the nitride semiconductor layer only.
The formation of a nitride semiconductor layer on a different base substrate of sapphire, etc. with large lattice mismatch is usually achieved not by the epitaxial growth of a nitride semiconductor coherent with the lattice of the base substrate, but by growing crystal nuclei in many points on the base substrate to such an extent that they are integrated to a continuous film. Accordingly, a film obtained by integrating such nitride semiconductor crystals does not have a lattice plane completely parallel to that of the substrate (not completely flat surface), resulting in slight deviations of crystal orientations therebetween. It is common to use a diffraction half width determined by an X-ray diffraction method as a method for evaluating crystal quality such as the deviations of crystal orientations, etc.
The quality of nitride semiconductor substrates is conventionally evaluated by a half width of an X-ray rocking curve in a {0002} or {0004} diffraction plane, and the smaller the half width, the higher the quality of the nitride semiconductor. For instance, Japanese patent 3,184,717 discloses a GaN single crystal as thick as 80 μm or more having a half-width of 5 to 250 seconds in a two-crystal X-ray rocking curve, which serves as a substrate, on which a GaN semiconductor crystal layer is grown. JP 10-053495 A discloses a nitride single crystal of 10 mm or more in both length and width and 300 μm or more in thickness, or 20 mm or more in length and 10 μm or more in diameter, which has a half width of 5 minutes or less in an X-ray diffraction rocking curve.